1. Field of the Invention
The present invention relates to a vibration driven motor or actuator for relatively moving a vibration member and a member urged against the vibration member by a travelling vibration wave generated in the vibration member and, more particularly, to a material of a sliding member in an urging contact portion.
2. Related Background Art
The principle of a vibration driven motor or actuator utilizing a travelling vibration wave will be briefly described below. A vibration member (stator) is constituted by fixing two groups each of a plurality of piezoelectric elements arranged in the circumferential direction on one surface of a ring-shaped elastic member having a total length corresponding to an integer multiple of a given length .lambda. and formed of an elastic material. In each group, the piezoelectric elements are arranged at a pitch of .lambda./2 to have opposite polarities so as to alternately exhibit opposite expansion/contraction characteristics, and the two groups of elements are arranged to have a shift corresponding to an odd-number multiple of .lambda./2 therebetween. Electrode films are provided to the two groups of piezoelectric elements. When an AC voltage is applied to either group (to be referred to as an A phase hereinafter) alone, the vibration member generates, over the entire circumference thereof, a standing wave (wavelength .lambda.) of a bending vibration having loop positions at the central points of the piezoelectric elements in the two groups, and at points separated therefrom at every .lambda./2 intervals, and having node positions at the central points between two each adjacent loop positions. When an AC voltage is applied to the other group (to be referred to as a B phase hereinafter) alone, a standing wave is similarly generated, but the loop and node positions of the wave are shifted by .lambda./4 from the standing wave generated by the A phase. When AC voltages having the same frequency and a temporal phase difference of 90.degree. therebetween are simultaneously applied to the A and B phases, since the standing waves of the two phases are synthesized, a traveling wave (wavelength .lambda.) of a bending vibration vibrating in the circumferential direction is generated in the vibration member. At this time, each point on the elastic member having a given thickness makes an elliptic motion. Therefore, when, e.g., a ring-shaped movable member (rotor) is urged against one surface of the elastic member, the movable member is rotated upon reception of the circumferential frictional force from the elastic member. It is known that when a plurality of radial grooves are formed in the circumferential direction on a surface, opposite to the piezoelectric element fixing surface, of the vibration member so as to increase the number of circumferential components of the elliptic motion, a neutral plane of a vibration is moved toward the piezoelectric element fixing surface, and the rotational speed is increased if the amplitude remains the same, thus remarkably improving motor efficiency. These grooves also have an effect of receiving a wear powder.
Based on the above-mentioned principle, the vibration driven motor or actuator has the following merits.
1) The motor has a holding torque in a non-energization state, and does not cause hunting.
2) Since the motor has a small inertia and a large driving torque, the rising and falling times of rotation are short (mechanical time constant is small).
3) Since all points on the circumference generate equal driving force, no cogging occurs.
With these merits, the vibration driven motor or actuator is suitable for high-precision rotation and high-precision positioning. However, since the motor utilizes the frictional force and the resonance of vibrations, it also suffers from the following demerits.
1) Since a sliding portion is worn, motor performance changes due to a change in frictional surface over time.
2) Since the resonance state and the coefficient of friction depend on temperature, temperature characteristics of motor performance are poor.
3) Since the heat generation amount is large relative to the volume of the motor, a considerable temperature rise occurs, and an adhered layer may be softened or damaged (peeled).
Therefore, in consideration of these merits and demerits, aluminum, phosphor bronze, stainless steel, or the like is used as needed as a material of the motor so as to satisfy the following requirements:
1) The motor must have high wear-resistance characteristics (to maintain motor performance for a long period of time).
2) The motor must have a high heat conductivity (to improve heat dissipation, thereby preventing a change in motor characteristics upon a change in temperature and a thermal damage to the motor).
3) The elastic member must have a thermal expansion coefficient close to that of an electro-mechanical conversion element (e.g., a piezoelectric element) (to prevent a damage to an adhered layer caused upon a change in temperature).
4) The movable member must have a light weight (to decrease the inertia).
In order to prevent a temperature rise as much as possible, a material having a high heat conductivity is preferable, and in order to realize a compact, lightweight structure and, especially, to decrease the inertia, a material having a low density is preferable. As a material having both these characteristics, an aluminum material is preferable.
As a material used in combination with the above-mentioned material, there are proposed composite resins causing a relatively small wear, and resins added with fillers having a reinforcement effect such as a fiber, a whisker, and the like, or added with fillers having a lubrication effect such as polytetrafluoroethylene ( PTFE ), molybdenum disulfide, graphite, and the like.
Since PTFE has a very low coefficient of friction even in a non-lubrication state, it is popularly used as a sliding material. PTFE molecules are transferred to a member contacting thereto due to the laminar peeling effect caused by a molecular coupling state, and cause friction between the PTFE molecules themselves, thus obtaining a stable low coefficient of friction. At the same time, the PTFE itself is worn considerably, and easily causes a creep. Therefore, the PTFE has a demerit of poor start performance after it is left for a while. As countermeasures against a wear and creep, PTFE filled with glass fiber, carbon fiber, aromatic polyester powder, polyimide powder, or the like has been proposed. However, since most of such fillers have certain hardness, a member to be brought into contact with the PTFE must be selected to have certain surface hardness so as to prevent a wear.
However, when the vibration driven motor is slid while a normal Cu-, Mg-, or Si-based aluminum material is in contact with the above-mentioned filled PTFE, aluminum as the base material is worn under the attack of the filler, and a wear powder of the aluminum is produced between the sliding surfaces. For this reason, transfer of the PTFE is disturbed, and only a wear occurs. Therefore, the sliding surface of the aluminum is subjected to various hardening treatments to prevent production of a metal wear powder.
However, in order to perform such a hardening treatment, pre- and post-surface treatments are required, resulting in a disadvantage in terms of cost. In addition, a hardened layer comprising an oxide film formed by plating or anodic oxidation easily falls off from a sliding portion, and the fallen powder is caught in the sliding portion, thus often causing a severe wear.